What is Gray, from a physics POV? Quora explains how white and black colors fit into the spectrum of visible light. It explains that white is all colors together while black is the lack of color.
So, where is Gray? Gray is the mix of all-colors and no-color!! What does that mean? Can somebody explain Gray, the mysterious color? 
 A: All colors are combinations of three wavelengths of visible light, red, green and blue.


*

*White = All Red + All Green + All Blue

*Black = No Red + No Green + No Blue

*Grey = unsurprisingly Half Red + Half Green + Half Blue!


Read this Wikipedia Page: https://en.wikipedia.org/wiki/RGB_color_model
A: All colors are only in the mind. Light has a mix of wavelengths, but it doesn't have color until someone sees it. 
When light enters the eye, it hits rods and cones in the retina. Cones are color receptors. There are three kinds. Each kind is sensitive to a range of wavelengths. Color is the result of stimulation of the cones, and additional processing in the brain. 

The image is from The Color-Sensitive Cones at HyperPhysics. Copyright by C. R. Nave, Georgia State University. A good starting link is the Light and Vision page. 
Loosely, the sensors are sensitive to long, medium, and short wavelengths. The ranges overlap. Most light, even single wavelength laser light, stimulates more than one. The graph shows which are stimulated by single wavelength light at different wavelengths. The colors we see are determined by the mix of stimulations. The bottom of the graph gives names of colors for single wavelength light. 
Grey is not on the list. Grey requires a mix of wavelengths that stimulate the three types more or less equally. So do black (very little stimulation) and white (more).
There is more to it than that. The perception of color is affected by colors around it. There are photographs where two different patches reflect the same light. But the colors we perceive are different because of the surroundings. For example, see the Checker Shadow illusion. 

By Original by Edward H. Adelson, this file by Gustavb [Copyrighted free use], via Wikimedia Commons
Also no wavelength will stimulate only the "Green" cones. They are always stimulated in combination with other cones. I once read it is possible to stimulate them with a probe. The person saw a color he had never seen before. I wish I could find a link. Quora might be a good place to start. 
